The variable aperture of the two mentioned lens make them less bright optically than comparable fixed aperture choices.

Brighter optics also, of course - allow for faster AF speed.

The 70-200 f/2.8 has brighter optics, allowing for a 1.6 stop advantage at 200mm. The brighter optics allow for faster, more reliable AF as well. The 'VR' feature will do zilch for the dirtbike races. Theres a variety of different 70-200 f/2.8's out there.